1. Field of the Invention
The present invention relates to a switching power supply unit transforming an input AC voltage obtained by switching an input DC voltage by a transformer, and outputting an output DC voltage obtained by rectifying and smoothing the transformed output AC voltage, and particularly relates to a switching power supply unit preferably applicable using high switching frequency.
2. Description of the Related Art
Hitherto, various types of switching power supply units have been proposed and provided for practical use. As one of the units, a type is given, in which an input DC voltage from a high-voltage battery is switched by switching operation of a switching circuit connected to an input winding of a transformer, an input AC voltage obtained by switching is inputted into the input winding of the transformer, and output AC voltage transformed by the transformer is drawn out from an output winding of the transformer, as described in Japanese Unexamined Patent Publication Nos. 2002-369528 and 2001-314080. A voltage induced on the output winding in association with the switching operation of the switching circuit is rectified by a rectifier circuit, the rectified voltage is converted to an output DC voltage by a smoothing circuit, and the smoothed voltage is outputted.
In a switching power supply unit of this type, a switching element such as MOS-FET (Metal-Oxide Semiconductor Field-Effect Transistor) is used for the switching circuit for size reduction. In such a switching circuit, the switching element performs switching operation at high frequency such as 100 kHz or more, therefore an input AC voltage obtained by the switching is a high-frequency voltage. There has been a difficulty that when such a high-frequency input AC voltage is inputted into a transformer, AC resistance of the transformer is increased due to a skin effect or a proximity effect, consequently heating is caused, leading to reduction in efficiency. Thus, to decrease AC resistance of the transformer, for example, a structure where section area of a winding is reduced so that current is not concentrated only on a surface of the winding, or a structure where primary windings and secondary windings are stacked in an alternate (sandwiched) manner so that windings having the same current direction are not close to one another, is used for the transformer (Japanese Unexamined Patent Publication No. 6-38523).